15.1 Pulsed Radar.
In part 14 we saw how Spatial Interferometry can be used to determine the bearing or direction to a signal source. RADAR (RAdio Direction-finding And Ranging) is a competing technique, widely used for detecting both the bearing and range of objects. Radar systems have come into widespread use during the last 50 years. More recently LIDAR systems — which use laser beams in place of radio/microwave ones — have begun to be used. Here we'll talk about radar, but all the following comments also apply to Lidar.
The simplest form of radar is illustrated in figure 15.1. A radar system includes a transmitter which emits a series of EM pulses. These pulses are radiated in a directional pattern using a suitable high-gain antenna. The radar then uses a receiver, usually connected to the same antenna, to detect any returned pulses being reflected from objects appearing in the antenna's field of view. The transmitter, antenna, and receiver are linked using a circulator. This ensures that all (ideally!) the transmitter power is sent to the antenna whilst all the reflected power returned to the antenna is sent to the receiver.
The form of the circulator depends upon the power level, signal frequency, etc. Figure 15.2 illustrates one example, a Free Space Quasi-Optical Circulator using a Faraday Rotator. This is based on the optical properties of magnetised ferrite materials. The material acts as a non-reciprocal device. The ferrite sheet rotates the polarisation of a plane-polarised EM wave through 45° as it passes through. This means that the transmitted power is rotated through 45° before being radiated. The returned power collected by the antenna is rotated through a further 45°. It therefore emerges from the ferrite sheet polarised at 90° to the field produced originally by the transmitter. Hence it is reflected by the polariser towards the receiver.
Radar systems determine the range of a reflecting target by measuring the time delay between the emission of a pulse and the return of a reflection. Bearing can be determined using interferometric methods, but in the simplest systems it is determined just by using a reasonably directional beam. (Obviously, a target outside the field of view illuminated by the radar won't produce a reflection.)
Knowing the time delay, , between pulse emission and the arrival of the reflected return we can say that the target range, R, should be
where c is the speed of light. One of the main problems which arises with simple pulsed radars is range cell ambiguity. This occurs because the radar emits a regular stream of pulses at a rate of, say, p per second. A target will therefore reflect a steam of pulses which return at the same rate. This presents the radar system with a problem when comparing the emitted and returned pulse streams. Is each returned pulse a reflection of the ‘most recent’ emitted pulse? Or does it come from a more distant target, and is a reflection of an earlier pulse? In principle, the target can be any one of a series of ranges
where . In effect, the radar's field of view is divided up into a series of cells, each having a ‘depth’, . To try an overcome this problem we can do one of two things:
- Reduce the pulse rate so that the first cell () is so deep that a target beyond it will be too far away to produce a detectable return.
- Vary the pulse rate and attempt to work out which pulse produces which reflection. In practical systems both approaches can be used together.
This can often let us overcome the problem, but at the expense of degrading the system's performance in other ways. For example, the sensitivity of the radar is usually increased by integrating the return patterns from success pulses. Reducing the rate and varying it means this will take longer, hence lowering the radar's sensitivity.
Another basic problem of radar stems from the fact that power must travel both to and from the target. Consider a target placed at a range, R, from an antenna of gain, G, operating at a signal wavelength, . The peak power level of each pulse is . Using the same arguments as in lectures 7 & 8 we can say that the power density (or flux) at the target will be
It is conventional to describe the target's overall reflectivity in terms of a Radar Cross Section (RCS), . This is defined such that
where is the total power reflected/scattered by the target which is assumed to be re-emitted isotropically. The target therefore behaves, so far as the radar is concerned, as an omnidirectional source radiating a total power, . Hence the peak pulse flux returned to the radar antenna will be
From lecture 8 we know that an antenna of gain, G, will have an effective area
hence the peak pulse power level reaching the radar receiver will be
This expression is a form of the Radar Equation. It has the same importance in radar as the Link Gain Equation in signal transmission. The most significant feature of this expression is that — unlike the link gain — the power returned to the radar falls as the fourth power of the range. This means that to double the maximum range at which a given target could be just be detected we would have to increase the transmitted power by a factor of 16. (Assuming we don't alter anything else.) As a result, long range radar systems often have to emit extremely high pulse power levels. This can be a problem in many situations. For example, microwave power levels in excess of kilowatts have a tendency to fry anyone who walks into the beam! They can also have other undesirable environmental effects. Another obvious drawback is the waste of energy radiating high power pulses out into space.
Another problem in a military context is that the radar emission acts as a beacon which shouts “I'm looking at you!” to anyone in the field of view with a suitable receiver. This fact was exploited by USA ‘ferret’ (not ‘ferrite’, this time!) aircraft in the Gulf War. These launched missiles specifically designed to passively fly down radar beams and zap the radar! A considerable amount of money has also been spent on developing ‘low-cross-section’ aircraft, etc, such as the F117 Stealth Fighter. These are designed to be as near-invisible as possible to conventional radars. Hence radar is of limited value given a distant, ‘weak’, ‘uncooperative’, or even ‘hostile’ target.
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